Learn how to use SetCPU to modify the default CPU settings on an Android phone (Note: you’ll need root access). It can not only overclock the processor to speed up the phone, but it can also underclock it to save battery life.

What’s noteworthy about SetCPU is that you can set it different scenarios for it to react to.  For example, when battery levels reach certain thresholds or even when it’s a certain temperature outside.  Although it costs $1.99,  you can accomplish similar feats for free by rooting your phone.